This invention relates to flow metering apparatus of the type wherein an elongated object in a stream of flowing fluid produces a wake including a distinct pattern of vortices, referred to as Karman's vortices, which are alternately shed from opposite sides thereof in a periodic manner. There is a definite relationship between frequency f of shedding of the vortices, the dimensions of the object and the velocity V of the fluid stream. For example, consider the geometry which has been the object of most studies of the shedding phenomenon, viz., a right circular cylinder of diameter D. The shedding frequency f is expressed by EQU f = KV/D (1)
where K is a constant when flow is within a certain range of velocity V. Thus, velocity can be determined by measuring the frequency f of the generation of the vortices. For a general discussion of vortex generation, reference may be made to Boundary Layer Theory by H. Schlickting, 4th Edition, McGraw-Hill Book Co., Inc., pps. 27-34. That text indicates that for Reynolds numbers above about 600, the Strouhal number is constant and the relationship expressed by equation 1 will be valid. This linearity between Reynolds number and Strouhal number prevails for Reynolds numbers up to about 4 .times. 10.sup.5 at which point the distinct pattern of vortices is replaced by random eddys. Vortex generating elements having noncircular cross-sections are taught in U.S. Pat. No. 3,572,117 issued Mar. 23, 1971 to A. E. Rodely and U.S. Pat No. 3,693,438 issued Sept. 26, 1972 to H. Yamasaki et al. The sharp edges of the vortex generating elements of the latter patent are said to improve the correspondence of vortex production rate with flow velocity over wide conditions of flow.
Various techniques, both electrical and mechanical, have been employed for producing electrical signals responsive to the rate of vortex production. To avoid the use of moving parts, electrical transducers are usually preferred. The vortex generating element itself sometimes contains passages through which an alternating fluid flows corresponding to the alternate shedding of vortices by the vortex shedding element. This type of vortex shedding element further contains means responsive to the alternating flow of fluid to produce an electrical output. Such shedding elements are costly and complicated because of the passages and flow sensing elements disposed therein. Moreover, the cross-sectional area of such shedding elements may be greater than desired because of the volume of the contents thereof, such elements thereby excessively impeding the flow of fluid through the channel in which they are disposed.
A simple and reliable flow sensing arrangement is disclosed in Paper No. 2-16-187 entitled "The Vortex Shedding Flow Meter"by D. S. White et al. presented at the 1971 Symposium on Flow-Its Measurement and Control in Science and Industry. The flow meter disclosed in that paper, which has been employed in the metering of numerous types of fluids and gases, comprises an elongated bluff body vortex shedding element having a frontal surface containing two thermisters, one on each side of the center thereof. For that flow meter to operate successfully the shedding element must have a frontal width which is an appreciable fraction, typically approximately one-third, of the diameter of the pipe across which it is placed. The thermisters occupy only a small fraction of the area of the frontal surface. The large area occupied by the frontal surface significantly impedes the flow of fluid, thereby causing a large pressure drop.